I've created a model that iterates through a folder to find file geodatabases, then exports data changes messages from each file geodatabase using the Export Data Change Message tool. For simplicity, the name each checkout is the same as the name of the geodatabase.
I've used an iterator to iterate through each of the file geodatabases in the folder, to get a Workspace variable representing the file geodatabase, and a string variable representing the name of the geodatabase. If I use the string variable representing the name of the geodatabase as the input to the Replica parameter in the Export Data Change Message tool, the tool will not validate and the model will fail. If I manually enter the name of the replica, then the model executes without error.